Method for quantitative determination of hydrogen peroxide using potentiometric titration

ABSTRACT

An electrochemical potentiometric titration method that entails titration of a known volume of a catholyte containing an unknown amount of hydrogen peroxide in a titration cell having two electrodes, a platinum working electrode and a silver/silver chloride reference electrode. A known concentration of a titrant is added to the catholyte in the titration cell. Simultaneously, as the titrant is added the potential between the working electrode and the reference electrode is monitored. The point at which all of the hydrogen peroxide has been consumed is signaled when the cell potential changes abruptly. Since the concentration of the titrant is already known, the amount of titrant added (concentration multiplied by volume) is directly related to the amount of hydrogen peroxide consumed. The concentration of hydrogen peroxide is calculated from the volume of catholyte and the moles of hydrogen peroxide.

This application is a divisional of pending prior U.S. patentapplication Ser. No. 11/183,311, U.S. Pat. No. 7,534,394, filed on Jul.11, 2005 and claims the benefit under 35 U.S.C. §121 of the priorapplication's filing date.

CROSS REFERENCE TO OTHER RELATED APPLICATIONS

Not applicable.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to titration methods, and morespecifically to a potentiometric titration method for a quantitativedetermination of hydrogen peroxide.

(2) Description of the Prior Art

There continues to be a need for energy sources with a high energydensity. In particular, there is a need for high energy density energysources that can power unmanned undersea vehicles. Such energy sourceswhen used to power such vehicles are required to have an energy densitygreater than 600 Wh kg⁻¹. They also need to have long endurance andquiet operation. Additionally, they must be relatively inexpensive,environmentally friendly, safe to operate, reusable, capable of a longshelf life and not prone to spontaneous chemical or electrochemicaldischarge.

The zinc silver oxide (Zn/AgO) electrochemical couple has served as abenchmark energy source for undersea applications. Because of its lowenergy density, however, it is not suitable for unmanned underseavehicles whose energy density requirements are seven times those of theZn/AgO electrochemical couple.

In an effort to fabricate power sources for unmanned undersea vehiclewith increased energy density (over zinc-based power sources), researchhas been directed towards semi fuel cells (as one of several high energydensity power sources being considered). Semi fuel cells normallyconsist of a metal anode, such as magnesium (Mg) and a catholyte such ashydrogen peroxide (H₂O₂). In general the performance and health of thesetypes of semi fuel cells are a function of the quantity of hydrogenperoxide in the catholyte. The key to achieving a high energy densityfor these types of semi fuel cells lies in the efficient usage of thehydrogen peroxide. The electrochemical processes during cell dischargeare:Anode: Mg->Mg²⁺+2e ⁻  (1)Cathode: H₂O₂+2H⁺+2e ⁻->2H₂O  (2)The voltage at the cathode and the total semi fuel cell voltage aredirectly related to the concentration of hydrogen peroxide in thecatholyte according to the Nernst equation:E=E ⁰+(0.0591*log([H₂O₂]*[H⁺]²))/2  (3)where E is the half cell voltage at the cathode, E⁰ is the standardvoltage at unit activity of H₂O₂ and H⁺, and [H₂O₂] and [H⁺] are themolar concentrations of peroxide and protons respectively. Equation (3)shows that as the peroxide concentration decreases so does the cellvoltage.

It is important to directly monitor and control the hydrogen peroxideconcentration [H₂O₂], because the concentration is used to assess thefunctional condition and performance of the semi-fuel cell. For example,if the hydrogen peroxide concentration differs significantly fromexpected levels for a given semi fuel cell load, then the pumpcontrolling the hydrogen peroxide input can be directed to increase ordecrease the amount of hydrogen peroxide being pumped into the semi fuelcell.

In a laboratory environment, measurement of hydrogen peroxideconcentration in a semi fuel cell is performed using a colorimetrictitration method. In this method, a solution of unknown peroxideconcentration is colored with a small amount of indicator material suchas iron(II) 1,10 phenanthroline. Then, a chemical of knownconcentration, typically cerium (IV) in sulfuric acid solution, (thetitrant solution) is added that reacts with peroxide. When the solutionturns clear, all of the hydrogen peroxide has been consumed. There is a2:1 correlation between the number of titrant reactant moleculesconsumed during the titration and the number of hydrogen peroxidemolecules initially present in the solution when cerium (IV) is used.The concentration of hydrogen peroxide can be determined using thiscorrelation. This method is not suitable for use in an unmanned underseavehicle, however, because it requires visible detection of a colorchange by a human operator. Currently there is no automated means forquantifying the concentration of hydrogen peroxide in a semi fuel cellonboard an unmanned undersea vehicle.

What is needed is a method of quantifying the concentration of hydrogenperoxide in a semi fuel cell catholyte that is automated and can provideconcentration data that can be interpreted by a digital processor.

SUMMARY OF THE INVENTION

It is a general purpose and object of the present invention to establisha method of quantifying the concentration of hydrogen peroxide in a semifuel cell catholyte that is automated and can provide concentration datathat can be interpreted by a computer.

This object is accomplished by employing an electrochemicalpotentiometric titration method. The method entails titration of a knownvolume of a catholyte containing an unknown amount of hydrogen peroxidein a titration cell having two electrodes, a platinum working electrodeand a silver/silver chloride reference electrode. A known concentrationof a titrant is added to the known volume of catholyte in the titrationcell. Simultaneously, as the titrant is added the potential between theworking electrode and the reference electrode is monitored. The point atwhich all of the hydrogen peroxide has been consumed is signaled whenthe cell potential changes abruptly. Since the concentration of thetitrant is already known, the amount of titrant added (concentrationmultiplied by volume) is directly related to the amount of hydrogenperoxide consumed. The concentration of hydrogen peroxide is calculatedfrom the volume of catholyte and the moles of hydrogen peroxide.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and many of the attendantadvantages thereto will be readily appreciated as the same becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a diagram illustrating the apparatus of the present invention;

FIG. 2 is a diagram illustrating flow injection analysis system of thepresent invention; and

FIG. 3 is a graph of dE/dV versus titrant as recorded by the injectionanalysis system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 there is illustrated a diagram of the inventionwhere an electrochemical titration cell 10 contains a certain volume ofcatholyte 12. Inside the titration cell 10 are two electrodes, workingelectrode 14 and reference electrode 16. Also contained inside thetitration cell is a mechanical stir bar 18. In the preferred embodiment,the titration cell will be relatively small to conserve volume when usedonboard an unmanned undersea vehicle. In the preferred embodiment, theworking electrode 14 is made of platinum, and the reference electrode 16is made of silver/silver chloride. A titrant solution 20 is introducedinto the titration cell 10. In the preferred embodiment, the titrantsolution 20 is a solution of Ce⁴⁺. The potential between the workingelectrode and the reference electrode is measured once the titrantsolution 20 is introduced. In the preferred embodiment, the potential ismeasured by means of a potentiostat/galvanostat 22.

The chemical reactions occurring in the titration cell are shown inEquations 4-6:H₂O₂-->2e−+2H++O₂  (4)Ce⁴⁺ +e ⁻-->Ce³⁺.  (5)H₂O₂+Ce⁴⁺-->2H⁺+O₂+Ce³⁺  (6)The addition of Ce⁴⁺ into the catholyte oxidizes the hydrogen peroxide.During the addition of Ce⁴⁺, the cell potential will be controlled bythe H₂O₂/O₂ redox couple. Immediately following consumption of all theperoxide, the cell potential will shift to that of the Ce⁴⁺/Ce³⁺ redoxcouple. This abrupt change in the cell potential signals the end pointof the titration and can be used by a computer to calculate the molarityof the hydrogen peroxide.

In a preferred implementation of the invention, a flow injectionanalysis system 24 is used for on-line analysis as illustrated in FIG.2. A micro-pump 26 will be connected to the catholyte chamber 28 of asemi fuel cell. The micro-pump 26 will remove a small fixed volume ofcatholyte and fill a fixed volume sample loop 30. The loop 30 emptiesinto a dilution chamber 32 containing a known volume of electrolyte thatdoes not contain H₂O₂ to dilute the small fixed volume of catholyte. Amicro-pump 27 will then fill a second fixed volume sample loop 34 withthe diluted sample of the catholyte. This diluted sample will be emptiedinto a titration cell 10 containing the reference electrode 16 and theworking electrode 14. The electrodes are connected to a combinedprogrammable digital processing unit 36 a and high impedance voltmeter36 b. The digital processing unit 36 a also controls a micro-burette 38that introduces the titrant into the titration cell 10 at a fixed rateas a stirring device 18 mixes the titrant and diluted sample. Thedigital processing unit 36 a receives the readings from the voltmeter 36b and performs the calculation dE/dV, where dE is the change in cellpotential and dV is the change in volume of titrant from the previousdata point. The endpoint of the titration is signaled when the slope ofthis graph changes from positive to negative as illustrated in FIG. 3.At this point the digital processing unit 36 a is programmed to stop themicro-burette 38 from introducing any more titrant into the titrationcell 10. Based on the volume of titrant that was delivered up to theendpoint, and because all of the volumes are fixed, the digitalprocessing unit 36 a is programmed to calculate the concentration ofhydrogen peroxide in the original catholyte sample.

In a laboratory experiment, 20 micro liters of catholyte was diluted to20 milliliters in a 50-milliliter dilution chamber containing 40 g/L ofsodium chloride. The H₂O₂ concentration in the catholyte was determinedto be 0.105 moles per liter using the calorimetric cerium (IV) titrationmethod. The diluted catholyte was then placed in a titration cell andthe cell potential was measured. A Ce⁴⁺ titrant solution that was0.001366 M was then titrated into a titration cell and the cellpotential was measured 45 seconds after each addition of titrant. Agraph of dE/dV versus titrant added is illustrated in FIG. 3. The graphshows the sharp change in dE/dV at 3.00 milliliters, which is the endpoint of the titration. The calculation of the hydrogen peroxidemolarity is as follows:(0.00300 L of Ce⁴⁺)*(0.001366 moles of Ce⁴⁺/one liter of Ce⁴⁺solution)*(1 mole H₂O₂/2 moles Ce⁴⁺)/20×10⁻⁶ L of catholyte=0.102 molesof H₂O₂ per liter of solution.The error of the measurement is acceptable at 2.9%.

The advantages of the present invention over the prior art areautonomous/automated control of hydrogen peroxide, H₂O₂, concentrationto assess the functional condition (health) and performance of ahydrogen peroxide, H₂O₂, based fuel cell.

Obviously many modifications and variations of the present invention maybecome apparent in light of the above teachings. For example theyinclude various reference electrodes such as the saturate calomelelectrode, various non corroding electrode materials such as gold orpalladium, various dilution ratios depending on titration cell volume,expected peroxide concentration, etc, various types of electronicinstrumentation to perform the measurement and acquire and process thedata, and different analysis methods to determine the endpoint such assecond derivative plot.

In light of the above, it is therefore understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

1. A method for determining the concentration of hydrogen peroxide in asolution, comprising the steps of: measuring a fixed volume of thesolution; diluting said fixed volume of the solution with an electrolytesolution; measuring a fixed volume of the diluted solution; depositingsaid fixed volume of the diluted solution into a titration cellcontaining a reference electrode and a working electrode; connecting thereference electrode and working electrode to a voltmeter; introducing atitrant into the titration cell at a fixed rate of volume; stirring thetitrant and the fixed volume of the diluted solution as the titrant isintroduced into the titration cell; measuring the cell potential of thetitration cell across the reference electrode and working electrode;periodically performing a calculation of the differential in cellpotential expressed as dE/dV, where E is the cell potential and V is thevolume of titrant introduced; stopping the introduction of titrant intothe titration cell when the differential in cell potential abruptlychanges; and calculating the concentration of hydrogen peroxide in thefixed volume of the solution based on the volume of titrant that wasintroduced into the titration cell.
 2. A method in accordance with claim1 wherein the step of measuring a fixed volume of the solution comprisespumping a volume of solution into a fixed volume sample loop.
 3. Amethod in accordance with claim 1 wherein said reference electrode iscomposed of silver/silver chloride.
 4. A method in accordance with claim1 wherein said working electrode is composed of platinum.
 5. The methodin accordance with claim 1 wherein the solution is a liquid catholytesolution of a semi fuel cell contained in the catholyte chamber of saidsemi fuel cell.